This invention relates to parallel path Coriolis mass flow rate meters and, more particularly, to an improved drive means for such a meter which permits it to be used in a wide variety of dangerous atmospheres, including hydrogen.
Coriolis mass flow rate meters are well known and are described in U.S. Pat. Nos. Re. 31,450, dated Nov. 29, 1983 to J. E. Smith and 4,422,338 dated Dec. 27, 1983 to J. E. Smith. A parallel path Coriolis mass flow rate meter is described in U.S. Pat. No. 4,491,025 dated Jan. 1, 1985 to J. E. Smith et. al., with all of these patents assigned to Micro Motion, Inc., the assignee of the present application. The detailed construction and operation of parallel path mass flow meters is set forth in Instruction Manual, Micro Motion Model "D" Mass Flow Meters dated December 1985 which is incorporated herein by reference. In general, the drive means used in a Coriolis mass flow rate meter comprises a coil and permanent magnet arrangement. A cylindrical ALNICO magnet is mounted on a beam having holes therein for closely receiving the magnet. The beam is then fixed to one of the flow tubes of the flow meter by conventional techniques such as welding or brazing. The longitudinal axis of the magnet is coincident with the centerline of a coil mounted on a second beam provided on the other parallel flow tube. One end of the magnet extends into the interior opening of the coil. The size of the magnet, the diameter and the number of windings of the coil is dependent on the size of the flow tubes that are to be driven. The coil is driven with a square wave or since wave driver amplifier circuit. The driver circuit is configured as a open loop servo circuit. Back-to-back zener diodes can be used to limit the inductive discharge spike of the coil to keep the energy level of the coil low to maintain an intrinsically safe rating. However, these diodes will limit the amount of drive current to the coil.
With the foregoing construction of mass flow meters, the flow of viscous liquids through the flow tubes can present problems because the damping of the flow tubes increases and, at times, the drive circuit will not keep the tubes in oscillation. Thus, it would be advantageous to have a means for increasing the drive power available. To increase the drive power, more turns of wire in the drive coil or larger permanent magnets in the drive can be used, but these are not satisfactory solutions. Increasing the magnet size increases the total mass added to the flow tubes which detracts from the balanced system. Additional mass also increases the momentum of the system and this makes the flow tube less responsive to the Coriolis forces. Additional mass also introduces other vibration modes into the system. To compensate for the increased mass of a drive assembly, stiffening of the mounting beam and additional mass further out from the centerline of the drive assembly mounting is required which, in turn, increases the total weight of the system and so on.
Adding more turns of wire increases the inductance of the drive coil and the inductance increases as the square of the length of the wire. Resistance increases linearly. A greater coil resistivity means that less current is necessary to drive the coil but since inductance increases at a square rate, the coil has a much larger stored energy capability. The stored energy restricts the applications into which the flow meter can be placed. In this regard, reference is made to Article 500 of the National Electrical Code (NEC) which will be referred to hereinafter. To enable a meter to be certified for Class I, Groups A and B (a hydrogen environment), referred to hereinafter, the total stored energy of the drive coil cannot exceed certain limits.
Limiting the stored energy of a drive coil means that it is deliberately designed to be inefficient, and this is contrary to usual design practices which stress efficiency. In addition, the mass constraints imposed limit the amount of iron and magnet that can be used and this in turn effects the volume of air gap and amount of wire in the coil.